Asked once at a party why he was wearing a face of dejection, the Hollywood actor Cary Grant confided despairingly: "It's the constant strain of trying to be Cary Grant."

Spanish monk/playwright Tirso de Molina's drama Jealous of Herself (La Celosa de si Misma) from c. 1622 plays with this idea in its hard look at the nature of identity, masculine desire and the problems faced by people - here a woman - compelled to meet idealistic expectations, reflected in a glass through which they are perceived darkly. Just one of these themes would be sufficient to underpin a 90-minute drama (no interval); the presence of a trio provides a dense bedrock, supporting Tirso's reputation as a playwright of intellect, though one prone to swaddle his thematic investigations in a wrapper of sardonic comedy, perhaps with a view to avoiding the blue pencil of the censor or defer an interdict by his religious superiors.

NowNowTheatre's production was billed as set "in the labyrinthine streets of 17th century Madrid". Well, maybe; the authentically Iberian black-and-white colour theme confronts the audience straightaway in the form of the giant chequerboard carpeting the floor and crawling up one wall - a Spanish patio, no doubt, and naturally a chess game in love suggests itself, especially given that the World Chess Championship was reaching a crucial phase in New York as the play rolled. For a chequerboard and briefly later a jalousie balcony to stand for a network of streets and alleys is a bit of a stretch, but set designer Harriet Bourhill had provided an arresting motif. Glowing neon 'Gents' and 'Ladies' signs also confronted the audience, another identity reference, though shouldn't they have read 'Caballeros' and 'Señoras'?

The action commenced with an ensemble of mannequin-types tiddling in with tiny steps to the catchy song 'Magdalena', home-composed by Alice Boyd, and this group of dancers plus small band and singer popped up later in interludes and most notably in the stunning coda to the play, reminiscent of the dance by the Globe Theatre players after every South Bank show. These interludes were a major asset - cleverly choreographed by Emmy Everest-Phillips and skilfully interpreted (Anushka Chakravarti and James Tibbles especially caught the eye). The heart of the drama was then exposed as our engaged-to-be-married hero Melchor and his confidential companion Ventura got themselves in a tangle over the women they willed into their lives, the former taking an ethereal, fantastical approach to ideal love while Ventura, sceptical of his friend's fancies, keeps his own feet square on the ground as he launches on a never-ending stream of bawdy talk and double-entendres, greeted with wild enthusiasm by some of the audience and relative reserve by others. Just how much of this humour is in the 17th century original I don't know, but we can suppose that post-grad adapter Sarah Grunnah has aimed to convey its essential tone.

The modernity of Tirso's material is extraordinary; as the dust from the gay marriage debate in Britain has yet fully to settle, and now the ins and outs of gender re-assignment are on the lips of progressive types everywhere, you could argue that his concern here with the nature of sexual attraction and jealousy, and his questioning of female passivity in the mating game are even more important, being more mainstream.

This was a stimulating, painstaking production, with its 16-strong crew more numerous than the cast. Director Ell Potter got the balance of comedy and thoughtfulness just right. Movement on stage and scene changes were super-smooth. The lighting design from Adam Marshall and Louise Drognat-Landre was striking; it didn't merely ring in the changes but marked moments where the optimism/pessimism of the lovers' trysts waxed and waned. Casting was universally strong. As our bewildered hero, Finlay Stroud retained his sang froid and emoted confidently. As the object of his desire, Rebecca Hamilton looked fine in her pine-tree shaped outfit, though I thought she lacked a bit of attack and tended to gabble a little. Kate Weir as Angela, the 'other woman' was excellent - sexily provocative, alternately icy and fiery. Joe Peden's Ventura, a Sancho Panza or even a Baldrick figure as he put down grandiloquence and stuck to the nitty-gritty, was truly a delight in a performance of endless comic invention - speech, movement, reaction, all pitch-perfect.

The first-night of audience of 75 only two-thirds filled the venue - a pity. This is precisely the kind of unknown dramatic gem that Oxford students should be digging out, and when given, as here, a first-rate production, it demands to be seen.